The invention relates to a machine and a procedure for extracorporeal treatment of blood.
Specifically, though not exclusively, the invention can be usefully applied in treatments involving an ultrafiltration of liquid through a semipermeable membrane, for example treatments for kidney failure such as pure ultrafiltration, hemofiltration, hemodialysis, and hemodiafiltration.
Extra-corporeal blood treatments generally involve removing blood from a patient, transporting it externally of the human body and reintroducing it to the patient once treated.
Treatments for kidney failure comprise, in general, both addition and/or removal of solutes from the blood, and removal of liquid.
As is well known, dialysis treatment is usually done by first removing blood from the patient and conducting it internally of a dialysis filter in which an exchange of solutes and liquid is performed through a semi-permeable membrane.
Various different typologies for dialysis treatment are known.
In pure ultrafiltration, the liquid to be removed is taken from the blood by pressure-gradient ultrafiltration through a semipermeable membrane, and is conveyed into a non-blood chamber of the filter. The solutes to be eliminated are drawn by convection along with the liquid, the convection transport being a contemporary transport of a liquid part of the blood and a part of the substances contained therein through the membrane.
In hemofiltration treatments, part of the liquid present in the blood crosses the membrane by ultrafiltration, drawing along with it, by convection, a part of the substances present in the blood, as in ultrafiltration treatment; also, a substituting fluid is infused into the blood before and/or after the filter, and in any case before the blood is returned to the patient.
In hemodialysis treatments, a sterile fluid of a predetermined composition is introduced into the non-blood chamber of the filter. The removal of liquid from the blood through the semipermeable membrane occurs by pressure-gradient ultrafiltration. The solute exchange through the membrane is essentially achieved by diffusion due to the concentration gradient of the solutes. A small part of the solutes is removed by convection along with the ultrafiltered liquid. In hemodialysis treatments no substitution fluid is used.
In hemodiafiltration treatments, the blood and the sterile fluid exchange substances both by diffusion and by convection, as in hemodialysis; in addition, a substitution liquid is infused into the blood as in hemofiltration treatments. The liquid is removed by ultrafiltration.
All the above types of dialysis treatment enable removal of an excess of liquid from the patient's body. The total quantity of liquid removed from the start to the end of the treatment (total weight loss) is one of the most important parameters in the treatment, and is generally set at the start of treatment as a target.
A further important parameter is the total treatment time.
In some treatments it is usual to pre-set the quantity of liquid removed from the patient per unit of time (also known as the weight loss rate). Usually the weight loss rate is set to a constant value or to a predefined profile. In any case, if a total weight loss value is added to the weight loss rate, the treatment time is determined a priori.
We observe in passing that the weight loss rate is equal, in general, to the difference between the ultrafiltration rate through the filter membrane and the flow rate of the substitution liquid (infusion rate). Also, in the case of hemodialysis or hemodiafiltration, the ultrafiltration rate is the same as the difference between the used dialysis fluid rate at the outlet of the non-blood filter chamber and the fresh dialysis fluid rate at the inlet of the non-blood chamber, while in the case of pure ultrafiltration or hemofiltration, the ultrafiltration rate coincides with the used fluid rate at the outlet of the non-blood chamber of the filter.
In the above-cited case, in which the weight loss rate is determined at start of treatment, if the infusion rate is also pre-set, then consequently the ultrafiltration rate will also be predetermined, as the sum of the weight loss rate and the infusion rate. The ultrafiltration actuators will therefore be controlled so that the ultrafiltration rate will follow the pre-set progression.
This type of control is essentially based on one or more of the flow rates of the liquids circulating in the dialysis machine (fresh dialysis liquid, used dialysis liquid, substitution liquid).
Apart from controls based on the flow rates, another known control is based on the trans-membrane pressure on the semi-permeable membrane of the dialysis filter. In this case the total weight loss is usually pre-set (or another desired target, such as for example the total quantity of a certain metabolite to be removed from the patient), and the trans-membrane pressure is also pre-set, usually to a constant value for the whole treatment. The ultrafiltration actuators are controlled so that the pressure follows the pre-set progression. The treatment terminates when the overall weight loss (or the other target parameter) is reached. In an HF/HDF treatment and in trans-membrane pressure controlled mode, if the total patient weight loss and the total substitution volume infused to the patient are predetermined, then the treatment time is not predeterminable. It is possible to set a predetermined treatment time even in pressure controlled mode; however if the total patient ultrafiltration volume (or the total patient weight loss) is predetermined as well, the total substitution volume will not be predeterminable.
A further important parameter which is usually pre-set at the start of treatment is the flow rate of the blood removed from the patient, which crosses the extracorporeal circuit. The choice of this parameter depends on various factors, not least the state of the patient's vascular access. The blood flow rate considerably influences both the quantity of solutes exchangeable per time unit, and the quantity of liquid removable per time unit, so the treatment time largely depends on the blood flow rate, i.e. the time in which the prefixed target (weight loss or quantity of metabolite removed or volume of substituting liquid infused or others) is reached.
Normally the blood rate is a pre-set value determined by the operator. EP 0 711 182 describes however a case in which the blood flow rate is determined by the dialysis machine, at start of or during treatment. In the system of EP 0 711 182 the blood flow rate, which is removed from the patient and sent to a dialysis filter, is varied in a predetermined sequence, after which the system measures the concentration of a metabolite (for example urea or creatinine, or uric acid) in the used dialysis liquid downstream of the filter, in order to determine a profile of the metabolite concentration in relation to the blood flow rate. The system thus determines the value of the blood flow rate which corresponds to the maximum metabolite concentration, and automatically sets the blood flow rate at the determined value. The system has the aim of optimizing blood purification performance with relation to the metabolite during the dialysis treatment. However, the system of EP 0 711 182 is not able to optimize the weight loss rate or the ultrafiltration rate. In other words, it does not guarantee to minimize the treatment time in cases where the pre-set target is a determined weight loss in the patient or a determined volume of substituting liquid to infuse in the patient.
WO 01/08723 discloses a method of optimizing the operating point of a dialysis filter. In WO 01/08723 a dialysis machine is controlled by calculating a filtration factor FF=UFR/[Qb*(1−Hct)], in which UFR is the ultrafiltration rate, Qb is the blood flow rate, and Hct is the hematocrit (i.e. the concentration of red blood cells in the arterial blood), and by verification of whether the filtration factor FF is within an admissible range. If not, the ultrafiltration rate is altered so as to return the system to a non-critical operating point. If the filtration factor FF is acceptable, it is checked whether the ultrafiltration factor can be incremented to increase efficiency in conditions of safety. In particular WO 01/08723 teaches that, in the case of increased deviation between the calculated filtration factor FF and the maximum limit set (for example in the case of previous reduction of ultrafiltration), the operating conditions can be modified so as to increase the efficiency of filtration, in particular by increasing the ultrafiltration rate. This makes it possible to modify the operating conditions of the filter dynamically during the treatment, following any variations and fluctuations of the hematocrit in the course of treatment, to obtain conditions of safety and increased efficiency at every instant. However, the method for increasing filter efficiency disclosed by WO 01/08723 is susceptible of improvement.